1. Field of the Invention
The present invention relates to a circuit and an element, which improve an image quality of a display screen in an active matrix display unit, and more particularly to a circuit and a device in which a circuit having a thin-film transistor (TFT) is used as a switching element, and an active layer of the TFT is comprised of a silicon semiconductor which has been crystallized using a catalyst element that promotes the crystallization of amorphous silicon.
2. Description of the Related Art
The active matrix display unit is directed to a display unit which is structured in such a manner that a switching element is provided for each of pixels, and a signal which is supplied through a video signal line is supplied to each of pixels by the switching element, thereby being capable of performing a distinct display of a larger capacitance than that of a simple matrix display unit. Up to now, a TFT using an amorphous silicon semiconductor has been used as a switching element. However, the operating speed of the TFT using a crystalline silicon semiconductor is at least ten times as high as that of the TFT using a conventional amorphous silicon semiconductor with the result that it is proper for a large-capacitance display and has been recently developed from this viewpoint. However, the crystalline silicon semiconductor has suffered from several problems.
A first problem resides in the crystallization of silicon. The crystalline silicon is obtained by crystallizing amorphous silicon. Two conventional methods have been known for crystallizing amorphous silicon, one of which is a method of crystallizing amorphous silicon with the irradiation of a laser beam or the like in a moment, which is called an optical annealing. This method suffers from such a problem that the reproducibility and the mass-productivity are poor because a laser oscillator having a stabilized large energy is not obtained.
Another method is called a thermal annealing technique or a solid-phase growing technique, which conducts thermal annealing usually at a temperature of 600.degree. C. or higher, to grow amorphous silicon in a solid phase, thereby crystallizing amorphous silicon. In the method, a period of time required for crystallization depends on an annealing temperature, and crystallization has been finished within one hour at a high temperature of the degree of 1000.degree. C. However, a substrate which is useful to such a high temperature is made of nothing other than quartz, which leads to an increase in the costs of the substrate. Also, the crystallization of the silicon film obtained is not excellent.
On the contrary, in the annealing at about 600.degree. C. that enables a lot of boro-silicate glasses to be used, a silicon film which is excellent in crystallization is obtained, however, a period of time required for crystallization is 24 hours or more, which leads to a problem from the viewpoint of the mass productivity.
A second problem resides in that a leak current (off-state current) becomes large when a reverse bias voltage is applied to a gate electrode in the TFT using crystalline silicon. This is considered to be caused by a grain boundary, and is of a most serious problem in the manufacturing of the active matrix display unit using crystalline silicon.
In the case of an n-channel TFT, an off-state current when V.sub.GS is biased negatively is regulated by a current that flows in a p-n junction which is formed between a p-type layer which is induced on the surface of a semiconductor thin film and a n-type layer in a source region and a drain region. Because a lot of traps exist in the semiconductor thin film (in particular, grain boundary), the p-n junction is incomplete to the degree that a junction leak current is liable to flow thereinto. The off-state current is increased as the gate electrode is biased negatively. This is because the carrier density of the p-type layer formed on the surface of the semiconductor thin film is increased so that an energy barrier of the p-n junction is narrowed in width, with the results that an electric field is concentrated and the junction leak current is increased.
The off-state current thus produced greatly depends on a source-to-drain voltage. For example, there has been known that the off-state current is actively increased as a voltage applied between the source and the drain of the TFT becomes large. In other words, comparing a case where a voltage of 5 V is applied between the source and the drain with a case in which a voltage of 10 V is applied therebetween, the off-state current of the latter may not be twice as large as that of the former, but may be ten times or 100 times. Also, such a non-linearly depends on the gate voltage. In general, when the value of the reverse bias of the gate electrode is large (a large minus voltage in the n-channel type), a difference between the former and the latter is remarkable.
As to the above-mentioned first problem, the inventors have found that the crystallization of amorphous silicon can been promoted by the addition of a little amount of nickel, platinum, iron, cobalt, palladium, etc., (Japanese Patent Unexamined Publication No. Hei 6-244104). Those elements to be added is called a catalyst element, and as a result, the crystallization could be performed through thermal annealing typically at 550.degree. C. for four hours or at a lower temperature for a less period of time. In addition, in the conventional thermal annealing technique, amorphous silicon having a thickness of less than 1000 .ANG. could be hardly crystallized. However, there has been found that using the catalyst element enables even amorphous silicon having a thickness of less than 1000 .ANG., typically 300 to 800 .ANG. to be satisfactorily crystallized.
As a result of the research conducted by the inventors, it has been found that, in the case of manufacturing the TFT using silicon which has been crystallized using those catalyst elements, the residual density of the catalyst elements in silicon is preferably set to 1.times.10.sup.15 to 1.times.10.sup.19 atoms/cm.sup.3 from the viewpoints of the crystallizing process, the characteristic and the reliability.
The first problem has been solved in this way. However, the second problem remains unsolved. Inversely, the crystal growth of the silicon film which has been crystallized using the catalyst element progresses in the form of a needle (in the conventional thermal annealing technique, it is in the form of a grain), and the length of the crystal is large to the degree of several .mu.m or more (in the conventional thermal annealing technique, it is 1 .mu.m or less). For that reason, there newly arises such a problem that TFT characteristic is largely effected by the grain boundary of the crystal, and the dispersion of an off-state current is large. The off-state current has been typically dispersed three figures of from 1000 pA to 1 pA.
FIG. 2A is a schematic diagram showing a conventional example of an active matrix display unit. A region 204 surrounded by a broken line in the figure represents a display region in which a TFT 201 is arranged in the form of a matrix. A wiring connected to the source electrode of the TFT 201 represents an image data signal line 206, and a wiring connected to the gate electrode of the TFT 201 represents a line select signal line 205.
The drive principle of that circuit is that a line is selected by inputting pulses to the respective line select signal lines of a N-th line, a (N+1)th line and a (N+2)th line while shifting a timing of inputting the pulse bit by bit as shown in FIG. 12.
In the circuit shown in FIG. 2A, the switching element is comprised of the TFT 201 and conducts the switching operation of data in accordance with a signal from the line select signal line 205 to thereby drive a liquid-crystal cell 203. An auxiliary capacity 202 is comprised of a capacity for reinforcing the capacitance of the liquid-crystal cell and is used for holding image data. For the purpose of uniformly displaying over the entire surface of the matrix, it is necessary to unify the characteristics of all the TFTs. More particularly, it is required that the off-state current is 10 pA or less, preferably 1 pA or less. If the TFT has the off-state current of 1000 pA, sufficient charges cannot be held, and a video signal becomes lost in a moment.
If the number of the above-mentioned defective TFTs is several in all the pixels, there arises no problem. However, if the number of the defective TFTs is several % of all the pixels, it is very hard to view the display. In particular, this phenomenon is remarkable in the TFT using a crystalline silicon obtained with a catalyst element as described above.
In order to solve this problem, for example, there has been proposed a method (multi-gate technique) of connecting TFTs in series as disclosed in Japanese Patent Examined Publication No. Hei 5-44195 and Japanese Patent Examined Publication No. Hei 5-44196. This method is intended to decrease the off-state current of the respective TFTs by reducing a voltage applied to the sources and drains of the respective TFTs. For example, in the case of connecting two TFTs in series as shown in FIG. 2B, a voltage applied to the sources and drains of the respective TFTs is made half. If the voltage applied to the sources and drains is made half, the off-state current becomes 1/10, 1/100 or the like as described above.
However, because the characteristic required for the image display of the liquid-crystal display unit became strict, it became hard to lower the off-state current as required even in the above-mentioned multi-gate technique. In other words, even though the number of gate electrodes (the number of transistors) is increased to three, four and five, the voltage applied to the sources and drains of the respective TFTs is reduced bit by bit to the degree of 1/3, 1/4 and 1/5. In order to make the voltage applied to the sources and drains 1/100, 100 gates have been required.
In other words, in the above-mentioned system, the effect is most remarkable when the number of gates is two. However, even though the number of gates is increased more than two, more effect could not be expected. In particular, in the TFT using a silicon film which has been crystallized with a catalyst element, as described above, the very large off-state current appears at a very high frequency. However, the above TFT was not useful to sufficiently cancel that influence.